Patent application No. WO 0033451 teaches a converter unit for converting two or more DC voltage levels from the input of the converter unit to a DC voltage on the output of the converter unit, wherein the converter unit comprises controllable switch means that are able to connect and disconnect the individual DC input voltage level for forming an oscillating signal, and wherein the converter unit comprises filtering means for low pass filtering of the oscillating, signal for forming the DC voltage on the output of the converter unit.
The converter unit discussed in WO 0033451, however, presents an inconvenience because it cannot connect to an AC supply source. Additionally, the converter unit is not capable of performing a gradual switch of supply source without supply loss, see the below description of a method. Nor is the converter unit capable of performing an adaptive switch in case of overload situations.
U.S. Pat. No. 5,751,564 discloses a switching power supply system which is able to connect two or more different power sources with different voltage levels, and can provide power even when the primary power source is low or completely absent. The output voltage is more constant than a conventional switching power supply, and the internal loss is also smaller. As a result, the back-up supply time is longer than that of a conventional UPS system. Finally, when used in a notebook computer, for example, there is no need to use an AC to DC adapter when connecting to an AC power supply, it being possible to connect the switching power supply directly to the AC power supply.
The ('564) system, however, is not capable of performing an uninterrupted switching between an AC supply source and a DC supply source.